Coal matrix deformation characteristics in the process of carbon dioxide displacing different gas saturation coal-bed methane

It is fundamental that changes in coal reservoir permeability are researched, in particular, the accurate determination of variations in the coal matrix caused by CO2 replacing CH4 at different gas saturation conditions. Based on the surface free energy, the extended Langmuir isothermal adsorption model, combined with CO2 replacing CH4 in experimental trials, and calling on the more general principles and characteristics of the field, mathematical models describing the coal matrix as it undergoes different processes such as CO2 injection and desorption were established. Combined with laboratory data about CO2 replacement under different methane saturation conditions, a law governing the variations in coal matrix CO2 replacement under different methane gas saturation conditions was obtained. The results showed that： in the injection process, the coal matrix expansion rate caused by CO2 or CH4 was exponentially increased with the CO2 pressure increase, the expansion caused by CO2 was far greater than the expansion caused by CH4 in the desorption process, the coal matrix shrinkage caused by CO2 or CH4 was exponentially increased with the pressure decrease, the shrinkage caused by CO2 was larger than the shrinkage caused by CH4 under the same pressure and different gas saturation, the total shrinkage in the desorption process in the coal matrix was greater than the total expansion in the injection process. At higher gas saturations, the total coal matrix shrinkage volume exceeded the total expansion corresponding to pressure points higher in the desorption process.

Molecular structure characterization of middle-high rank coal via^(13)C NMR,XPS,and FTIR spectroscopy

Elemental analysis,nuclear magnetic resonance carbon spectroscopy(^(13)C-NMR),X-ray photoelectron spectroscopy(XPS)and Fourier transform infrared spectroscopy(FTIR)experiments were carried out to determine the existence of aromatic structure,heteroatom structure and fat structure in coal.MS(materials studio)software was used to optimize and construct a 3D molecular structure model of coal.A method for establishing a coal molecular structure model was formed,which was“determination of key structures in coal,construction of planar molecular structure model,and optimization of three-dimensional molecular structure model”.The structural differences were compared and analyzed.The results show that with the increase of coal rank,the dehydrogenation of cycloalkanes in coal is continuously enhanced,and the content of heteroatoms in the aromatic ring decreases.The heteroatoms and branch chains in the coal are reduced,and the structure is more orderly and tight.The stability of the structure is determined by theπ-πinteraction between the aromatic rings in the nonbonding energy EN.Key Stretching Energy The size of EB determines how tight the structure is.The research results provide a method and reference for the study of the molecular structure of medium and high coal ranks.